An undercarriage is a part of an aircraft used to support the plane on the ground. It includes wheels, skids, or pontoons, along with supporting struts and gear. The undercarriage is an important part of the aircraft and becomes especially critical during takeoff and landing, when it is submitted to immense forces as the plane accelerates or decelerates. Regular aircraft maintenance includes inspection of the undercarriage and servicing of all moving parts to make sure it will function when needed.

There are several approaches to undercarriage design. Designers must consider the best way to distribute the weight of the aircraft without creating unnecessary drag with the undercarriage, while thinking about issues like potential interference with propellers or engines. Some designers have created retractable undercarriages, seen on a lot of commercial aircraft. Once the plane is in flight, the wheels are tucked up into the fuselage to make the plane more aerodynamic.

Sometimes, a single wheel is located under the front of the plane, with a set of two further back along the body. Other planes reverse this, with a single wheel under the tail and a set under the midbody. Especially large and heavy planes may need support for the wings while the plane is on the ground, with a set of wheels at the fuselage and another further out along the wings. In planes designed for water takeoffs and landings, pontoons or skids may be installed, and planes can also be mounted on skis for handling snow.

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In the case of the wheeled undercarriage, a common choice, the tires used must be regularly inspected and retreaded or replaced. With large, heavy planes, the amount of wear on the tires is considerable and can be exacerbated by harsh conditions like inclement weather. The tires are usually inspected after every landing as part of a check of the plane to make sure they are safe for use. Planes kept in storage for a long time need to be inspected and cleared for use, looking for wear associated with sitting idle on a tarmac, like insect damage and corrosion.

Inspection of aircraft landing gear can be especially complex with retracting undercarriage. Technicians need to make sure the storage area for the undercarriage is clear, and they check for issues like fouled lines and wires, while lubricating and inspecting the moving parts. Another issue can be electrical shorts or mechanical problems that may be hard to identify when a plane is on the ground.

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